Periodically Agenda on Mac does not accept files (primarily PDFs) via drag-and-drop.
For example, this morning after starting my computer and Agenda, I tried to drag a pdf from Finder into an existing note in Agenda. The cursor would not appear. I tried dragging from Devonthink and the same thing happened. I worked a while on other things and then tried again and the Agenda accepted the same file from the same Finder window without trouble.
This has happended on my iMac27 with High Sierra and my MAcbook Pro with Mojave.